I recently got an Eico 720, and it's been working great on CW. I have a home theater receiver that has 3 dead channels, but on the 2 good ones I can get 125W into a 16 ohm speaker. Judging by the schematic for the Eico 730 modulator I want to have about a 7.5K impedance into the 720. I looked through the parts I have and behold! A plate transformer! I hooked the amp to the filament windings and one of the plate windings (670V CT) to the octal plug and....nothing. After some measuring of the HV winding into a 10K resistor I found I was getting 6W out, and turning the volume any higher caused the protection circuit to trip. Doing some calculating I found the amp was seeing about 2 ohms.

The next try was putting the amp into the 120V winding, and then connecting the two HV windings in series (total of about 1500V). I hooked it up and....I heard audio in the receiver! And it was clear! I was getting about 25W of modulation into the 720, which isn't going to fully modulate the carrier, but it certainly did the job. The issue was as soon as I tried to make a contact with someone across town the received audio on his end sounded terrible, and it was as if there was very little modulation at all.

Wouldn't I hear the lack of modulation in my receiver? Any ideas on what I should be checking? Am I missing something obvious?

A typical plate modulation transformer has a secondary impedance of around 5k to 7k depending on the PA tube(s). It is possible to use a plain vanilla power transformer or audio output transformer - except - you'll be running them backwards.

Typically a transformer is wound with the primary nearest the core so as to induce maximum flux in the laminations. In some cases the primary and secondary windings are 'stacked' equally around the core like two Pregnant Pi windings and would be preferable for this application. More often than not the secondary is wound on top of the primary. You should have a power transformer like that in your junque box with a 6 or 12 volt secondary and 120 VAC primary. Try the secondary across the amplifier with the primary in line with the B+ of the EICO PA section. Might work. Likewise a tube output transformer in the 7k range with a 3.2 to 8 Ohm secondary wired backwards might do something, but remember the pseudo-primary will be shielded from the core so I wouldn't expect much efficiency or a reliable impedance match when it's bass-ackwards. Also, with any modern lo-volt power transformer remember the Hi-Pot rating might be only 300 or 600 volts... Not the 1500 more common back in the day.

Ultimately you may end up keeping an eye open for an older PA amplifier with a nice beefy output transformer capable of 70 volt (600 Ohm) output. Research this and you'll find plate modulation is possible with the 70 volt winding in series with a second inductor. You'll find big iron in both tube and silicon PA amps and the worse shape the better as you're buying it for the transformer, so uglier is both cheaper and nicer. Sorta'. Iron like that runs into serious coin if you try to replicate the matching EICO modulator using new parts.

Amps like this tend to go cheap 'cuz they're not 100 Watts or better, but 35 Watts RMS is more than enough to drive the EICO to full modulation and it has mic pre-amps, tone controls plus a transformer isolated output. With an amp like this the 35 Watt rating is real and not BS "music power" so there's some reserve should you want to add a compressor circuit.

Power transformers are made with minimum air gaps, which is why they have the laminations interleaved. But it means that the core is more easily saturated by DC, and in that case, you will lose audio and likely get distortion.

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